So, with those three assumptions out of the way, we can start to do calculations.

I used the Tron Cycle conversion chart from on the wiki.

Using TC 301 (Anon's creation date) as the date of the Coup in Tron Cycles, and November 3rd, 1989 as the final rez date of Flynn (The date from the Tron: Uprising opener), we get a start date for the Grid, by subtracting 1 TC from the date (According to the TRON: Evolution TRON files, they start at 1, not zero) and converting 300 TCs into days, then working the days backwards, accounting for the leap years 1984 and 1988.

November 5th, 1983 is the start date for the Grid.

From the same date for the Coup, we can figure out when Legacy happened, using the screensaver running on Flynn's terminal. The time given is 20 years, 11 months, 20 days, 16 hours, 22 minutes, 16 seconds, and some other random time that according to IMDB, updates 15 times per second. That came out to October 25, 2010.

I could not find any official evidence that Sam's pranks on ENCOM corresponded every year to the anniversary of Flynn's disappearance, what I believe is more likely is that he pulled something at big events, like the Space Paranoids Online and Encom OS12 releases.

Thus, The Grid was created November 5th, 1983, the Coup happened and Flynn disappeared November 3rd, 1989, and TRON: Legacy happened October 25th, 2010.

"...(W)hen we quantify electrical impulses, those speeds are over 1,000 greater - so, if life could exist in a purely digital form, that organism would experience 1,000 years for every one year of our time on earth. When I work on my special project, it's as if time stands still in the "real world." Sam's grandparents can sit with him while I toil for what seems like a week, but when I return, they barely notice I've gone."